1. Field of the Invention
This invention relates generally to magnetic head servo control systems and, more particularly, to disk drive position signal demodulator systems to determine the location of a read/write head relative to disk tracks.
2. Description of the Related Art
In conventional computer data storage systems having a rotating storage medium, such as a magnetic or magneto-optical disk, data is stored in a series of concentric or spiral tracks across the surface of the disk. A magnetic disk can comprise, for example, a disk substrate having a surface on which a magnetic material is deposited. The data stored on a disk is represented as a series of variations in magnetic orientation of the disk magnetic material. The variations in magnetic orientation, generally comprising reversals of magnetic flux, represent binary digits of ones and zeros that in turn represent data. The binary digits must be read from and recorded onto the disk surface by a magnetic transducer comprising a read/write head that is suspended over the disk surface in close proximity to the disk. That is, the read/write head can produce and detect variations in magnetic orientation of the magnetic material as the disk rotates relative to the head.
Conventionally, the read/write head is mounted on a disk arm that is moved across the disk by a servo. A disk drive servo control system controls movement of the disk arm across the surface of the disk to move the read/write head from data track to data track and, once over a selected track, to maintain the head in a path over the centerline of the selected track. Maintaining the head centered over a track facilitates accurate reading and recording of data in the track.
The servo control system maintains the read/write head in a position centered over a track by reading servo information recorded onto the disk surface. The servo information comprises a servo pattern of high frequency magnetic flux transitions, generally flux reversals, that are pre-recorded in disk servo tracks. The servo pattern flux reversals are distributed about the centerline of each servo track and, when read by a servo read head, provide a signal that is a function of the location and orientation of flux reversals in the track located beneath the servo head. The signal thereby provides an indication of the direction and extent of head movement required to maintain the head centered about the track.
A servo read head, which can be the same head used for reading the binary data or can be a dedicated servo pattern read head, detects the servo pattern as the disk rotates and also produces servo track information. The servo track information includes track identification data that provides the number of the track from which the information was produced and also includes an analog servo signal that provides information on the position of the servo read head relative to the track. The analog servo signal is demodulated by servo control system circuitry to produce a position error signal that is used to control the disk arm servo. In this way, the servo control system detects the number of the track over which the servo read head is positioned and controls movement of the servo head relative to the track. Those skilled in the art will appreciate that the servo read head is located in a fixed position relative to the data read/write head and therefore controlling the position of the servo head also controls the position of the data head.
There are a variety of methods for providing servo track information to a disk servo control system. In a method referred to as the dedicated servo method, the entire surface of one side of a disk is pre-recorded with dedicated servo track information. A servo head is positioned over the dedicated servo disk surface in a fixed relationship relative to data read/write heads positioned over one or more other data disk surfaces. The position of the servo head relative to the dedicated disk surface is used to indicate the position of the multiple data read/write heads relative to their respective disk surfaces. The dedicated servo method is most often used with multiple disk systems in which a servo head of a single dedicated servo disk surface controls movement of corresponding data read/write heads of a multiple-platter disk drive. It should be apparent that a dedicated servo system for a single disk system would use one-half of the available disk surface area for servo information only and therefore would not be especially efficient.
Another method of providing servo track information is known as the sector servo method. In the sector servo method, each disk surface includes servo track information and binary data recorded in concentric or spiral tracks. The tracks on a sector servo disk surface are divided into radial sectors having a short servo track information area followed by a data area. The servo track information area includes a sector marker, track identification data, and a servo burst pattern. The sector marker indicates to the data read/write head that servo information immediately follows in the track. Again, the servo read head can be the same head used for reading data or can be a separate, dedicated servo head.
The sector servo method is more efficient than the dedicated servo method for low profile disk drives with fewer disks in the configuration, because a single read/write head can be used to obtain servo information and to read and record data from the disk and because less of the disk surface area is used for servo information as compared with the dedicated servo method. As users demand greater storage capacities from low profile disk systems, manufacturers provide less and less disk area for servo information by decreasing sector length and track width. To obtain the same amount of servo information in less disk area, the servo information must be recorded at higher and higher frequencies. The higher frequencies increase the difficulty of writing and reading the servo information.
In both the dedicated servo and sector servo methods, the analog servo signal is produced as the servo pattern is read from the disk and is demodulated to provide an indication of the servo head position relative to the inside and outside diameters of the disk. The demodulated servo signal is commonly referred to as a position error signal (PES) and is used to generate a corrective input signal that is applied to the read/write head positioning servo. The remaining description assumes the sector servo system, but the manner in which the sector servo description can be applied to dedicated servo systems should be clear to those skilled in the art.
Conventionally, the phase-encoded servo pattern read by the servo read head comprises one or more radial and/or slant patterns formed by magnetic transitions aligned across the servo track in the disk radial direction. The phase-encoded servo pattern generally includes either an area or burst of radial lines that are followed by an area of angled or slanted magnetic transitions, or two sequential opposed slant patterns. A conventional combination radial line and slant phase pattern is illustrated in FIG. 1 and a conventional chevron-shaped, opposed slant pattern is illustrated in FIG. 2. Each dark stripe represents an area of magnetic flux transitions from one polarity to another. Thus, a servo read head following a path crossing the stripes will generate a sinusoidal analog servo signal having peaks and valleys that coincide with the stripes. The stripe patterns are said to be phase-encoded, because the phase difference between the analog servo signal generated when the servo head is over one pattern and the analog servo signal generated when the servo head is over the other pattern determines the radial distance of the head from the diameter of the disk.
In FIG. 1, for example, a portion of a combination radial/slanted servo pattern in a single servo track is shown comprising a radial portion 12 and a slanted portion 14. An exemplary on-track path followed by a servo read head is indicated by a horizontal line 16. The servo head position is determined by comparing the phase of a first analog servo signal produced by the servo read head when it is over the radial portion with the phase of a second analog servo signal produced when the servo read head is over the slant portion. In FIG. 2, the servo pattern includes a first slanted portion 20 having a plurality of slanted lines and a second slanted portion 22 having a plurality of slanted lines at right angles to the lines of the first portion 20. An exemplary on-track path followed by a servo read head is indicated by a horizontal line 24. As with the FIG. 1 pattern, servo head position information from the pattern illustrated in FIG. 2 is produced by comparing a servo signal produced from the first slant line portion 20 with a signal produced from the second, opposed slant line portion 22. An example of a servo system using such a pattern is described in U.S. Pat. No. 4,549,232 to Axmear et al., which is incorporated herein by this reference.
Typically, a servo control system for controlling the position of a servo read head includes a demodulator for servo signal sampling and for amplitude-type or phase-type detection. Such demodulators use passive and active discrete components for filtering, automatic gain control setting, and analog-to-digital conversion. Typical phase-type demodulator designs utilize zero-crossing phase detectors and produce a position error signal through a combination of phase counting, pulse counting, and averaging analog-to-digital conversion. That is, with every zero crossing of the sinusoidal analog servo signal, a pulse is generated in the demodulator. The delay between pulses generates an approximation of the phase difference between the servo signals generated by the two patterns. The manner of generating the approximation is well-known to those skilled in the art and requires no further explanation.
The various components used to implement the analog module for phase sampling and position detection can make it difficult to provide small-size electronic packages of reduced power consumption, especially for the increasingly popular small form factor (SFF) disk drives. In addition, the zero-crossing phase detection scheme provides very limited sampling points. Specifically, a zero-crossing phase detection scheme approximates a servo signal with only two sample points per cycle of the signal. This limited sampling can decrease the ability to minimize or otherwise eliminate noise from the servo signal.
Moreover, utilizing phase-encoded servo signal demodulation techniques can cause a non-linear demodulator output signal. That is, the phase difference function produced as the servo read head moves from the inside diameter of a servo track to the outside diameter of a servo track is not uniform so that the servo sensing gain obtained from a predetermined servo track radial movement at the edges of a track is not equal to the servo sensing gain obtained from radial movement at the center of a track. This complicates the design of the servo control system.
From the discussion above, it should be apparent that there is a need for a disk drive system with a servo pattern encodement and detection scheme that reduces overall circuit complexity and size and that lends itself to noise minimization techniques. The present invention fulfills this need.